This project consists of a 20-year prospective study of the population-at-risk for osteoporosis. Research subjects are 199 normal women, who were aged 35-45 at the start of the study six years ago. Observations are made once every five years and consist of comprehensive calcium kinetic, calcium metabolic and metabolic and physiolgical measurements, as well as radiogrammetric and photo densitometric measurement of bone mass. A total of 90 primary and derived variables is accumulated on each subject. The purposes of the project are: 1. To obtain comprehensive longitudinal data on calcium metabolism, bone metabolism, and related physiological functions in a group of untreated women from the perimenopausal period to the median onset time of osteoporotic symptoms; 2. Thereby to provide an exhaustive physiological profile both on women who develop symptomatic osteoporosis, and on those who do not, allowing characterization of the differences between them and of the factors influencing the differences; 3. Thereby to permit better identification of the osteoporosis-prone subset and to provide both a rational basis for prophylactic treatment and the identification of subjects who should be its target; 4. And finally, to characterize cross-sectional interrelationships between major physiological variables during this key era of osteoporosis development.